Thermosetting powder coating compositions are well known in the art and are widely used as paints and varnishes for coating electric appliances, bicycles, garden furniture, accessories for the automotive industry, and the like. Thermosetting powders consist of a mixture of a primary resin and a crosslinker, often called a hardener. The general approach associated with powder coating technology is to formulate a coating from solid components, mix them, disperse pigments (and other insoluble components) in a matrix of the major binder components, and pulverize the formulation into a powder. In so far as possible, each particle contains all of the ingredients in the formulation. The powder is applied to the substrate, usually metal, and fused to a continuous film by baking.
Powder coating technology offers a number of significant ecological advantages over liquid coatings. Film forming components of liquid paints include resin which have required organic solvents to provide the resins with suitable viscosities such that the paint can be applied by existing commercial application equipment. Use of organic solvents, however, raises at least two problems. In the past and potentially in the future, petrochemical shortages mitigate against the use of organic solvent in great volumes. Second, environmental concern mitigates against the use of organic solvents and requires such use be minimized.
Environmental concern has become increasingly important. This concern not only extends to preservation of the environment for its own sake, but extends to public safety as to both living and working conditions. Volatile organic emissions resulting from coating compositions which are applied and used by industry and by the consuming public are not only often unpleasant, but also contribute to photochemical smog. Governments have established regulations setting forth guidelines relating to VOCs which may be released to the atmosphere. The U.S. Environmental Protection Agency (EPA) has established guidelines limiting the amount of VOCs released to the atmosphere, such guidelines being scheduled for adoption or having been adopted by various states of the United States. Guidelines relating to VOCs, such as those of the EPA, and environmental concerns are particularly pertinent to the paint and industrial coating industry which uses organic solvents which are emitted into the atmosphere.
An important factor in the acceptance and growth of the powder coating industry has been their environmental acceptability. These types of coatings are essentially 100% nonvolatile, i.e., no solvents or other pollutants are given off during application or curing.
Powder coatings have distinct economic advantages over liquid, solvent-containing paints. The coating material is well utilized since only the powder in direct contact with the article is retained on the article, any excess powder being, in principle, entirely recoverable and reusable. No solvent storage, solvent dry off oven, or mixing room are required. Air from spray booths is filtered and returned to the room rather than exhausted to the outside. Moreover, less air from the baking oven is exhausted to the outside thus saving energy. Finally, disposal problems are lessened because there is no sludge from the spray booth wash system.
The use of powder coatings further provides advantages in terms of convenience and performance. Powder coating are more convenient to use as compared to other coating methods for many types of applications. They are ready to use, i.e., no thinning or dilution is required. Additionally, they are easily applied by unskilled operators and automatic systems because they do not run, drip, or sag as do liquid coatings.
Powder coatings provide a high level of performance. The reject rate is low, the finish tougher and more abrasion resistant, than most conventional paints. Thicker films provide electrical insulation, corrosion protection, and other functional properties. Powder coatings cover sharp edges for better corrosion protection.
Despite the many advantages associated with powder coatings, these compositions have a number of limitations. A major challenge in developing powder coatings is satisfying a combination of sometimes conflicting needs: (1) stability against sintering during storage, (2) coalescence and leveling at the lowest possible baking temperature, and (3) crosslinking at the lowest possible temperature in the least possible time. Further, the degree of flow and leveling must be balanced to achieve acceptable appearance and protective properties over the range of expected film thicknesses. Films that flow readily before crosslinking may have good appearance, but they may flow away from edges and corners, resulting in poor protection.
If the Tg of the coating is high enough, sintering can be avoided. However, coalescing and leveling at the lowest possible temperature are promoted by having the lowest possible Tg. Short baking time at low temperatures are possible if the resins are highly reactive and if the baking temperature is well above the Tg of the final crosslinked film. However, such compositions may crosslink prematurely during extrusion, and the rapid viscosity increase as the particles fuse in the oven limits the ability of the coating to coalesce and level.
Polyester powder coatings are a well known type of thermosetting coating which are typically formulated with epoxide compounds to yield powders which can be applied to various substrates by electrostatic spraying or fluidized bed and then cured by baking. Triglycidyl isocyanurate (TGIC) has been widely used as a crosslinker for carboxylic acid terminated polyesters. U.S. Pat. Nos. 5,006,612 and 4,740,580 describe powdered coating compositions which are polyester resins with carboxyl functionality for cross-linking with epoxy functional crosslinkers, such as TGIC. However, curing at temperatures below 140.degree. C. is not described and when the composition at the '612 patent is baked for 30 minutes at 138.degree. F., it develops a haze and not very good mechanical properties.
Further, U.S. Pat. No. 5,439,988 describes polyesters for the preparation of thermosetting powder coating compositions useful in varnishes and paints. The carboxyl-terminated polyester of the '988 patent is prepared using a two step process. In the first step a hydroxyl-terminated aliphatic polyester is prepared from 1,4-cyclohexanedicarboxylic acid, as the sole acid, and a cycloaliphatic diol, either alone or in admixture with aliphatic polyols, mixtures of neopentyl glycol and tri- or tetrahydric aliphatic polyols are preferred (column 4, lines 22-24). In the second step, this hydroxyl-terminated polyester is reacted with an aliphatic and/or aromatic dicarboxylic acid or the corresponding anhydride to bring about a chain extension and a carboxylation of the polyester, adipic and isophthalic acid are preferably used (column 4, lines 51-53). The carboxyl-terminated polyester is used together with a triglycidyl isocyanurate to provide thermosetting powder coating compositions. Curing is effected at a temperature from about 150 to 190.degree. C. in about 10 minutes in the presence of up to 1% catalyst (column 3, lines 21-23), including phosphonium salts. However, the problems associated with curing below 150.degree. C. are not addressed.
WO 93/04122 describes carboxyl group bearing polyesters that are formulated with epoxy compounds and phosphonium salts. Curing below about 180.degree. C. and problems associated with curing at lower temperatures are not described.
The problems associated with low temperature cure have been reviewed in a number of articles and patents. While catalyst can be used to reduce the curing temperature in a powder coating system, there are at least two major drawbacks. First, there is a risk of premature reaction during the extrusion process, which is carried out at a minimum temperature of 90-100.degree. C. Second, poor surface appearance of the applied film may result due to partial crosslinking before complete fusion of the powder. This results in heterogenous film formation which manifest itself as texture and orange peel in the applied film.
Blooming and poor mechanical properties are commonly associated with lower bake temperatures. One of the causes of blooming is the formation of a 22-member cyclic oligomer with a crystalline structure and melting point of 275-280.degree. C. ("Determination of the Chemical Nature of the "Blooming" Effect in Polyester Based Powder Coatings, 13th International Conference Nov. 15-17, 1993 Brussels by Hoechst Sara S.P.A.). This 22-member cyclic oligomer normally volatilizes at higher bake temperatures.
Typically, the Tg of the polyester resin must be maintained at a minimum value of about 55.degree. C. in order to avoid caking and sintering of the finished coating powder during storage (Loutz et al., Polymer Paint Journal 183(4341):584 (1993). Such high Tg values are associated with a high viscosity when the polyester is fused at 120-130.degree. C. Amorphous polyesters generally have a viscosity at 200.degree. C. of from 11-110 poise, which also hinders the formation of a homogenous film.